The present invention relates to a rubber composition for a base tread and a tire using the rubber composition, more specifically, a tire of a two-layer structure, which not only achieves of fuel consumption but also reduces deformation of an extruded tread in forming tires and assures uniformity after vulcanization, and a rubber composition for a base tread used for a base portion of a tread of the tire.
In recent years, demands for saving of resources and energy have been increasing throughout the world. In the course of developments of car tires and electric car tires which can decrease energy loss and decrease fuel consumption, various studies have been made specifically as to a tire tread structure and rubber materials used for the tread.
For instance, there are proposed ideas to save fuel consumption, such as a fuel saving tire which has a tread of two-layer structure comprising a cap portion and a base portion instead of a tread of a conventional one-layer structure and is provided with a fuel saving rubber on the base portion of the tread (JP-B-12244/1985, JP-B-45961/1988, and JP-B-33361/1989), and a styrene-butadiene co-polymer rubber (JP-A-100 112/1982 and JP-A-179212/1982) which is obtained by solution polymerization in an organic solvent with the use of organolithium compound as a catalyst and is used for the cap portion of the above-mentioned tread.
Another idea for reducing fuel consumption is the use of a rubber composition prepared by blending a small amount of a carbon black as a reinforcing material, in a rubber such as a natural rubber, synthesized isoprene rubber and butadiene rubber, used for the base portion of the tread having the above-mentioned two-layer structure.
Certainly the problems of tires having a conventional one-layer structure, such as increase in rolling resistance and lowering of grip characteristic, are solved and fuel saving can be achieved to a considerable extent. In cases of the above-mentioned fuel saving tires, fuel saving rubbers one employed for the base portion, styrene-butadiene copolymer rubber for the cap portion of the tread, and a rubber composition prepared by blending a small amount of a carbon black in a rubber such as a natural rubber, synthesized isoprene rubber and butadiene rubber is used for the base portion of the tread. However, contrarily there is a problem that uniformity of an overall shape of the tires becomes remarkably worse mainly due to non-uniformity of a base tread thickness.
Therefore further studies have been made as to how the above-mentioned lowering of uniformity of the overall tread shape occurs. It was found that there are some differences in a tread gauge distribution between a tire molded and vulcanized by the use of a nonvulcanized tread immediately after extension and cut, and a tire molded and vulcanized by the use of a nonvulcanized tread which has been allowed to stand for a while after extension. That is to say, when the nonvulcanized tread rubber is extruded and cut to a circumferential length for a tire and is allowed to stand, for instance, for from two or three hours to 24 hours, there occurs shrinkage at the cut portion in the extruding direction, a little expansion in the direction of width and in the direction of thickness. The extruded tread gauge changes due to such shrinkage. Furthermore it was confirmed that the degree of the shrinkage varies mainly with the components of a rubber composition being used for the base portion of the tread having a two-layer structure.
Therefore there is a desire to develop a tire and a rubber composition used for a base portion of a tread of the tire that not only reduced fuel consumption but also wherein tread gauge distribution scarcely changes and uniformity of shape after vulcanization is excellent, even if the tread rubber is allowed to stand for a while after extrusion and cutting, such that little shrinkage occurs, particularly on the base portion of the conventional tread having a two-layer structure.